This invention relates to apparatus for the conversion of video information from a radar receiver for display on a cathode ray tube (CRT). More particularly, the radar data is received in a polar coordinate format (R,.theta.) and converted to a cartesian coordinate format (X,Y) for display on a raster type display device.
The presentation of radially formatted Plan Position Indicator (PPI) type radar data at high resolution has been virtually the exclusive domain of cursive displays. Presentation of such radially formatted data on raster type displays requires a conversion of the scanning direction from radial or polar coordinates to X,Y coordinates; unfortunately, such a conversion cannot be conducted as a one-to-one memory cell exchange because a one-to-one correspondence between R,.theta. radial cells and X,Y raster cells does not exist.
The prior art attempts to perform the needed conversion from a polar to a cartesian coordinate format by the use of a "closest proximity" re-mapping scan converter approach. With this technique, sample cells of incoming radial data are mapped into the nearest corresponding X,Y addresses. However, the resultant scan converted image is at best marginal and the following deficiencies become evident: the detailed shape of video information is seriously deformed with a resultant loss of net resolution; all X,Y cells do not necessarily contain converted data, that is, black holes can occur within "target" boundaries; the radial intervals between active radials tend to form Moire patterns with the raster X,Y address which are very distracting to an operator; and the storage of converted data in final refresh memory format is complicated by the fact that it is not produced in normal X,Y scanning order. The present invention eliminates all of these deficiencies.
Another approach in the prior art performs a coordinate conversion on radar video input data corresponding to each azimuth angle of a transmitted radar beam. However, time consuming calculations comprising determining trigonometric functions and multiplications have to be performed requiring high speed arithmetic hardware instead of primarily using simple adders.
Other approaches in the prior art of scan converters have involved "horizontal smearing" or "tangential smearing" techniques for filling data gaps between radial lines and/or between regions on each radial. To avoid said gaps, the data for each particular region is used from surrounding regions to fill in the gaps. However, these approaches although probably sufficient for some low density and low resolution applications are not as precise and lack the conversion speed generally required for radar applications where each radial is converted when its data is available rather than waiting for all radial lines to be stored and then initiating a conversion process.